Anal. Chem. 1983, 55, 867-873
867
Multiwavelength-Selective Ionization of Organic Compounds in1 an Ion Mobility Spectrometer David M. Lubinan" and Me1 N. Kronlck Quanta-Ray, Inc., 1250 Charleston Road, Mountain View, Californla 94043
Laser multlphotlon lonlratlonIs used to demonstrate the posslblllty ob an added dlmenslonallty In analysls for plasma chromatography, Le., wavelength selectlvlty. I n resonant two-photon lonliratlon a molecule wlll lonlre If the two-photon energy Is greaterr than the lonlratlon potentlal of the molecule and there Is a rieal lntermedlate state resonant wlth the flrst photon. With Khls scheme a crude spectral selectlvlty Is posslble among groups of molecules wlth widely dlfferlng lonlratlon potentlals. This technlque may be useful for slmpllfylng the anallysls of mixtures of molecules In a plasma chromatograph and may aid In the separatlon and Identlflcatlon of molecules, In partlcular, Isomers. A wide range of useful data are collected and presented In an Ion moblllty spectrometer at elevated temperatures. I n addltlon, an ArF exclmer laser operating at 194 nm Is Introduced as a general lonlratlon souroe, capable of lonlrlng the vast majorlty of organlc compouinds.
In previous papers we introduced a unique method for producing ions for plasma chromatography (1-3). This method uses laser multiphoton ionization to ionize molecules directly under atmospheric conditions in an ion mobility spectrometer. The main advantage of using laser radiation as the ionization source is that i t minimizes the problem of nonspecific ionization. The conventional method of producing ions for plasma chromatography involves using a Ni-@source to initiate a series of ion-molecule reactions which transfer charge to the species under study (4-22). 'Thistechnique often produces data difficult to interpret since the ion-molecule reactions may cireate several different ion-molecule combinations with the trace compounds. The laser source permits direct ionization of molecules with ultraviolet (UV) light, thus obviating the need for the ion-molecule reactor. In addition, the multiphoton ionization (MPI) process can provide extremely efficient ionization of large organic molecules with only the molecular ion appearing when the laser operates a t sufficiently low intensity. Thus, only one peak appears in the ion mobility spectrum corresponding to each molecule ionized. In our work, wle are using UV radiation to produce resonant two-photon ionization. This process involves absorption of one UV photon to a real resonant state,followed by absorption of a second photon which will produce ionization if the energy of the two photons exceeds the ionization potential of the molecule. This process has been shown to be extremely efficient when it proceeds through a real resonant state. If no real state exists, bhen the process must occur through a very short livedl virtual state and the cross section for ionization becomes negligible. Most large molecules however do have strong absorptions between 350 and 200 nm, although the absorption spectra of these molecules are often broad and featureless at the elevated working temperature of an ion mobility spectrometer (IMS) due to the large number of Povibronic stpatespopulated. In addition, most large organic molecules have ionization potentials below 12 eV and are thus 0003-2700/83/0355-0867$01 S O / O
easily ionized with near-ultraviolet light (23-26). The above properties allow the possibility of an added dimensionality in analysis for plasma chromatography, Le., wavelength selectivity. Previous papers have illustrated the use of distinct spectral features for uniquely identifying several different molecules in a plasma chromatograph (PC), specifically isomers of xylene (2) and cresol (3). Further work has shown that this method is not generally applicable to large molecules whose spectra are often featureless at elevated temperatures (3). However, by use of the fact that different groups of molecules have widely differing ionization potentials, a crude spectral selectivity should be achievable. This can be potentially useful for simplifying the analysis of mixtures in a PC or for distinguishing groups with low ionization POtentials (IP) such as amines from groups with high IP's such as alcohols. This selectivity is not available with the present p source ionization technique. In addition, even if the laser energy is greater than two times the IP, the ionization efficiency will be low if no absorption is present. This feature can be further used as a crude form of spectral selectivity between compounds. The use of color selectivity can aid in unambigously identifying a peak whose mobility spectrum mlay present several possibilities in identification. This case may present itself for several compounds that have the same or similar mobilities. One problem of interest that the ion mobility spectromei%r can sometimes solve is the separation of isomers (1%21). The plasma chromatograph is essentially an atmospheric pressure drift tube which separates compounds on the basis of their drift times or characteristic mobility. The mobility of a compound typically decreases with ion mass, but it also depends on the size and shape of the ions (19). Bulky ions generally require a longer drift time than compact ions of similar structure (19). Thus, isomers of a compound oftlen have different reduced mobilities. Using selective wavelength ionization, however, we can demonstrate a further ability to distinguish several isomers which happen to have very different spectral properties. In this paper we extend our previous work (1)by presenting data on a wide range of organic molecules in order to dernonstrate the ability to selectively excite systems based upon a combination of their IP's and their spectral properties. In order to generate a variety of wavelengths in the region of interest, we have used a Nd:YAG pumped dye laser system as in former studies and have introduced the use of an excimer laser to this technique to produce light in the far-UV. In addition, we discuss the UV ionization of specific moleculles of interest and demonstrate the utility of this scheme in discerning information about mixtures of molecules.
EXPERIMENTAL SECTION The experimental setup is similar to that discussed in our previous work (1-3). The ion mobility spectrometer is a modified version of a commercial plasma chromatograph obtained from PCP, Inc., West Palm Beach, FL. The present setup can be operated either as a conventional plasma chromatograph with a Ni-p source or as a laser chromatograph by changing the bias circuit and introducing a laser beam through the cell window. The 0 1983 Amerlcan Chemical Society
868
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
Table I. Ionization as a Function of Wavelength
compound
mol wt
aniline n-meth ylaniline m-toluidine 2,4-lutidine p-n-butylaniline N,N-diethylaniline N,N, 3,5-tetramethylaniline N,N-dimethylaniline 2,4-dimethylaniline hexylamine diisopropylamine triethylamine tert-butylamine sec-butylamine n-butylamine me thylamine for mamide dimethylformamide benzene toluene xylene phenol cresol cyclohexane hexane indene pyridine pyrrole naphthalene azulene anthracene phenanthrene tetracene p-nitrotoluene 2,6-dinitrotoluene methanol ethanol 1-nonanol 1-octanol acetone benzophenone benzaldehyde p-dioxane ethyl formate
93 107 107 107 149 149 149 121 121 101 101 101 73 73 73 31 45 80 78 92 106 94 108 84 86 116 79 67 128 128 178 178 228 137 182 32 46 144 130 58 182 106 88 74
IP, eV
expected cutoff, nm
7.7 7.32 7.5 8.85 7.53 6.99 7.25 7.14 7.44
322 338 330 280 329 354 342 347 333
7.73 7.50 8.64 8.70 8.71 8.97 10.25 9.12 9.23 8.82 8.5 8.51 8.52 9.8 9.45 8.81 9.3 8.2 8.13 7.42 7.43 7.80 7.01 9.82
320 330 287 28 5 284 276 24 2 271 268 28 1 291 291 291 253 262 281 266 302 305 334 333 317 353 252
10.84 10.49
228 236
9.98 9.4 9.52 9.13 10.61
24 8 263 260 271 233
drift distance of the ions created by the Ni-P sowce is 8.0 cm while that for the laser source is 12.5 cm. This difference is a result of the grid structure necessary to pulse ions into the drift region as discussed previously ( I ) . Thus, all comparisons between figures presented for the two techniques must be multiplied by 12.5/8 to obtain comparable drift times from the /3 source to the laser technique. The injection pulse was typically 0.2 ms for the ions created by the P source. A pulse 0.5 ms in duration was used if a larger signal was required, although the longer pulse results in a loss of resolution. The laser pulse was similarly apertured by a 2-mm rectangular slit to maximize the resolution ( I ) . A decrease in the size of the laser beam perpendicular to the collector increases the resolution, while the size of the beam parallel to the collector does not affect the resolution. A rectangular slit is therefore used to maximize both the resolution and signal. For larger signals the slit was enlarged up to 5 mm. In cases where the signal was still too small, a quartz positive 30 cm focal length lens was used to bring the beam to a soft focus at 25 cm from the lens inside the PC at its center axis. Suprasil 1 windows (2.54 cm diameter) were necessary in order to transmit the UV laser radiation down to 194 nm. Other grades of quartz such as Optosil or Homosil will not transmit light at this wavelength. The apparatus was operated at 220 "C. The countercurrent flow of dry N2 was typically 600 cm3/min and was filtered with a molecular sieve trap before it entered the plasma chromatograph. The operating voltage of the power supply was set arbitrarily at 2725 V. Higher voltages could produce arcing at the elevated tem-
320 nm
310 nm
293 nm
280 nm
266 nm
249 nm
194 nm
W Y Y N Y Y Y Y Y W W N N N N N N N N N N N N N N W N N N N W N Y N N N N N N N
Y Y Y N Y Y Y Y Y W Y Y N N W N N N N N N N N N N Y N N N N Y N Y
Y Y Y W Y Y Y Y Y Y Y Y Y W Y N N N N W Y N
Y Y Y Y. Y Y Y Y Y Y Y Y Y Y Y N N W N Y Y Y Y N W Y W Y Y Y Y Y Y N N
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y N Y Y Y Y Y Y N Y Y Y Y Y Y Y Y Y N N N N N N
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y N Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y W N N N W W Y Y Y Y W
Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y
N N N
N
N
N
N
N N Y W Y Y W Y W Y N N N N N
N
N
N N N
N N
N N
N N N
W N N
N
N Y W N
peratures of the IMS, while lower voltages caused a spreading of the peaks due to the longer diffusion times. Two different laser systems were used in this study. The first laser source was the Quanta-Ray DCR-1A Nd:YAG laser used alone at its fourth harmonic (266 nm) or used at its second harmonic to pump various dyes in a Quanta-Ray PDL-1 dye laser. In order to produce UV light, the output from the dye laser was frequency doubled in a phase-matched KD*P crystal. This was performed for the various dyes with the Quanta-Ray WEX-1 wavelength extender device. The dyes used to produce the different wavelengths were as follows: DCM for 310 and 320 nm; Kiton Red for 293 nm; and Rhodamine 590 for 280 nm. The second laser source was a Lambda-Physik EMG 101 excimer laser (courtesy of the San Francisco Laser Center). This laser was used to produce light using KrF at 249 nm and ArF at 194 nm. The ion mobility signal was digitized into 800 points over a 40-ms interval using a DEC ADV-11 A/D interface to a DEC PDP-11 computer, The digitized signal was signal averaged over 1000 laser pulses. The resulting ion mobility spectrum was then displayed on an x-y recorder. The chemicals used in this study were from Aldrich Chemical Co. and were ordered for the maximum purity obtainable, preferably the gold label grade. The samples were prepared by the diffusion tube method (28). Diffusion tubes (Vici Metronics, Santa Clara, CA) with aperatures of either 0.5 or 0.2 cm and a length of 7.62 cm were used to produce samples in concentrations on the order of several parts per million. The samples were further
ANALYTIGAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
8 0.51
11 Figure 1. (a) Laser induced mobility spectrum of p-xylene and n methylaniline. The peak at 19.1 ms corresponds to p-xylene while that at 20.0 ms corresponds to n-methylanillne. The conditions under which this spectrum was recorded are as follows: = 266 nm, T = 220 OC,input laser energy = 0.1 mJ, laser beam slze = 2 mm X 6 mm rectangle, electric field = 170.3 V/cm, p-xylene concentration = 250 ppb, n-methyianiline concentratlon = 130 ppb, drift gas flow = 600 cm3/min. Kdxylene) = 2.13, K&-methylanlllne) = 2.03. (b) Laser induced mobiilty spectrum of p -xylene and n-methylaniline at X = 310 nm. Only the n-methylaniline peak appears. The condftions under which thls spectrum was recorded are the same as in (a) except that the input laser energy = 0.25 mJ. (c) Ion mobility spectrum of p-xylene and n-mdhylanliine produced with the Ni-/3 source. The grid pulse for injecting the ions into the drift region was 0.2 ms. The peak at 13.1 ms corrc3sponds to p-xylene and that at 13.6 ms to n methylaniline. Thae other relevant parameters are the same as in (a) and (b). K,(xylerie) = 1.99, KO@-methyianlline)= 1.91. Note the difference in K,(x:ylene) values for the two different lonlzatlon techniques as discussed in ref 3.
diluted by the drift gas to concentrations on the order of 30-200 ppb by flowing the carrier N2 gas slowly.
RESULTS AND DISCUSSION In Table I we present ionization results for 45 organic
a
869
Figure 2. (a) Laser Induced mobility spectrum of m-toluidine at 320 nm. The condltlons under which this spectrum was recorded are as follows: T =: 220 OC,Input laser energy = 0.35 mJ, laser beam size = 2 mm X 6 mm rectangle, electrlc field = 170.3 V/cm, mtoluidine concentration = 55 ppb, drift gas flow = 530 cm3/min. K,(m-toluidine) = 1.94. (b) Ion mobility spectrum of m-toluidine produced with the Ni-P source. The peak at 13.4 ms corresponds to m-toluidine, while the other peaks are residual contamination present when this spectrum was taken. The grid pulse for injecting the ions into the drift region was 0.2 ms. The other relevant parameters are the same as in (a), drift gas flow = 530 cm3/min. K,(m-toluidine) = 1.94.
compounds in an ion mobility spectrometer based on eight different wavelengths in the ultraviolet region. In most cases the compound either does not ionize (N) or ionizes easily (Y) as denoted in the table. However, there are some case13in which the ionization is weak or not detectable using the standard conditions discussed below but is reasonably strong with high laser powers or a softly focused beam. Typically, the signal obtained in the case of an easily ionizable compound -1 V. In the present system with an electrometer which has an output of 5 X lo9 VIA, a 1-V pulse of 350 ~s fwhm would be equivalent to a signal of -4.5 X lo5 ions. A signal which is CO.1 V in magnitude at the maximum power and concentration levels used in these studies has been arbitrarily defined as a weak signal in Table I. Essentially all compounds (can be ionized at almost any wavelength with enough power. Thus, this table holds at modest laser energies where mainly the molecular ion or MH+ is detected. The range of laser energy used in this table is between 0.5 and 2 mJ unfocused. Of course, the signal can be increased by raising the concentration of the substance under study. The concentrations used in this table range from 30 to 200 ppb. The high end of this range was used for compounds with weaker signals, while the low end was used for compounds with very large signah in order to minimize contamination of the device. Generally the concentration was kept a t 100 ppb. Concentrations above 1 ppm can contaminate the IMS for lengthly periods of time.
-
870
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6,MAY 1983
I
1
10-
I Figure 3. (a) Laser induced m o b i l i spectrum of l-octanol at X = 194 nm. The conditions under which this spectrum was recorded are as follows: T = 220 OC, input laser energy = 1 mJ, laser beam diameter = 3 mm, electric field = 170.3 V/cm, l-octanol concentration N 100 ppb, drlft gas flow F 560 cm3/min. K,(octanol) = 1.94. The small peaks at longer drift time are due to background in the IMS device. (b) Ion mobility spectrum of l-octanol produced wlth the Ni-(3 source. The grld pulse for injecting the ions into the drift region was 0.5 ms. The other relevant parameters are the same as In (a). K,(octanol) = 1.94.
The immediate conclusion that can be drawn from Table I is that compounds such as amines with relatively low ionization potentials ionize a t long wavelengths, while compounds with high ionization potentials such as alcohols need shorter wavelengths for ionization. In fact, the amines can often be distinguished from the aromatic hydrocarbons, alcohols, ketones, etc. by this crude wavelength selectivity. In addition, in a mixture the amine compounds would be selectively ionized relative to the other compounds. One example of this is shown in Figure 1which is a mixture of n-methylaniline and p-xylene. At 266 nm both compounds ionize and can be distinguished in this case by their different mobilities although the two peaks overlap partially. At 310 nm only the n-methylaniline peak appears under similar conditions, and the peak can be uniquely distinguished from p-xylene. This feature would be particularly useful for distinguishing n-methylaniline from another compound with a peak that could not be resolved from the n-methylaniline peak. A second example of this type of selectivity was demonstrated in ref 1,Figure 7. In this case we could observe ionization of naphthalene a t 280 nm. Toluene does ionize at 280 nm but is not observed here since the cross section for ionization is reduced compared to that a t 266 nm. This is due to a smaller optical absorption at 280 nm than at 266 nm. Thus, toluene was not observed a t the concentration shown in this figure. It should be noted that most of the aromatic hydrocarbons studied in Table I ionize strongly at 266 nm but that most alcohols, ketones, and aldehydes do not ionize unless
Flgure 4. (a) Laser induced mobility spectrum of hexane at X = 194 nm. The conditions under which this spectrum was recorded are as follows: T = 220 OC, input laser energy = 1 mJ, laser beam diameter = 3 mm, electric field = 170.3 V/cm, hexane concentration N 100 ppb, drift gas flow = 560 cm3/min. K,(hexane) = 2.10. The small peaks at longer drift time are due to background in the IMS device. (b) Ion mobility spectrum of hexane produced with the Ni-@source. The grld pulse for Injecting the ions into the drift region was 0.5 ms. The relevant parameters are the same as in (a). K,(hexane) = 2.15.
shorter wavelengths are used. A second important conclusion is that 194-nm laser radiation of sufficient intensity will ionize all large organics in this study. This was previously shown to be true for a large number of similar compounds in an ionization cell under vacuum (29). However, 194 nm still does not efficiently ionize NH3, etc. compared to the background gases such as N2,02, organic compounds under study. The other excimer laser line used at 249 nm also ionizes most organics of interest. Thus, an excimer laser with its discrete number of output wavelengths in the UV may be sufficient for use as a general ionization source for many analytical applications. This technology thus presents real analytical possibilities for laser produced ions in plasma chromatography. Present high peak power tunable laser systems are usually very expensive instruments which require high technical skill to operate. However, excimer lasers which can produce several UV wavelengths and high peak power pulses a t high repetition rates have become relatively inexpensive and are reasonably straightforward to use. Another important conclusion is that at the elevated temperature of an IMS many compounds continue to ionize a t longer wavelengths than their two-photon ionization limit would predict. This may be due to hot band absorption at elevated temperatures. Some examples of this effect are seen in compounds such as 2,4-lutidine, benzaldehyde, toluene, hexane, etc. Indene ionizes at 320 nm in the IMS at elevated temperatures although its two-photon ionization limit occurs at 281 nm. This is not true in a cell at room temperature. This
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6,MAY 1983 * 871
I
me maw 10-
I
208m*.F
Figure 5. Laser induced mobility spectrum of n-methylaniline at X = 310 nm. The conditions under which this spectrum was recorded are as follows: T = 220 O C , input laser energy = 1 mJ, laser beam diameter = 4 mm, electric field = 170.3 V/cm, n-methylaniline concentratitin = 130 ppb, drift gas flow = 600 cm3/min. K,(n-methyianliine) = 2-00.(b) Laser induced mobility spectrum of 2,4-lutidine at X = 310 nm. The parameters are tihe same as in (a): 2,4-iutidine concentration = 70 ppb, drift gas flow = 600 cm3/min. Note that no apparent signal is observed. (c) Laser induced mobility spectrum of m-toluidine at X = 310 nm. The parameters are the same as in (a): m-toluidine concentration =- 60 ppb, drift gas flow = 530 cm3/mln. K,(m-toluldine) = 1.94. (d) Laser induced mobility spectrum of 2,4-lutidineat A = 280 nm. The parameters are the s a m e as in (a) . . exceDt the input laser energy _.= 0.6 mJ: 2,4-lutidine concentration = 70 ppb, drift gas flow = 600 cm3/min. K0(2,4-lutldine) = 1.95.
appears to be true in many more compounds than can be shown in Table I. Benzene ionizes past 270 nm although no signal is expected at wavelengths longer than 266 nm. There are also compounds which may not ionize although the wavelength may be at shorter wavelengths than the twophoton limit. This may be due to a lack of absorption or to rapid radiationless processes a t elevated temperatures as discussed in the case of azulene and naphthalene in a previous publication (1). Table I presents an important compilation of data which cannot necessarily be predicted. The use of laser photoionization can simplify a spectrum relative to the [I source ionization. In the case of m-toluidine (Figure 2a) laser ionization at 320 nm produces only a single peak. At this wavelength not even contamination present in the IMS device which appears at 280 nm is observed. In the @ source spectrum (Figure 2b) taken right after the laser spectrum preslanted above, several peaks appear for mtoluidine. In this particular figure, contaminants from previous experiments are present and appear in the mobility spectrum. In the case of 1-octanol at 194 nm we observe only one peak (Figure 3a) as opposed to the @ source spectrum (Figure 3b) where a second peak is observed under the same conditions or as in the paper of Karasek et al. (17) where several peaks were observed. This was also observed in the case of hexane as shown in Figure 4. It has been shown (19)that isomers of alkyl amines can be distinguished bly their mobility spectra. In this study we present similar results for a set of alkyl amines and a set of aromatic amines. We indeed were able to distinguish these compounds by using plasma chromatography. However, wkh
laser ionization we can obtain another degree of selectivity. In the case of n-methylaniline, m-toluidine, and 2,4-lutidine, the former two will ionize at 320 nm while the latter will not (Figure 5 ) . Thus, laser color selectivity can aid in separating isomers. This crude separation was also possible in the case of the isomers hexylamine, diisopropylamine, and triethylamine where hexylamine ionizes weakly at 320 nm while the other two compounds ionize strongly (Figure 6). The separation of compounds according to IP can also be accomplished by using short wavelength UV radiation from a CW lamp to directly ionize organic compounds (26);however, we believe there are several advantages to using the laser multiphoton technique. Firstly, vacuum UV will not be transmitted through air at atmospheric pressure, so that the usable range of wavelength from lamp sources will be limited. Laser resonant two-photon ionization uses two visible or near-UV photons to achieve ionization so that transmission of the required wavelength through air is generally nod a problem. A second advantage is the gain in spectral information provided by going through an intermediate photon. A third advantage is the ability to easily collimate or focus the coherent radiation of the laser when desired. The most important advantage of laser MPI is the expected gain in sensitivity over the CW lamp technique. The ionization rate in the IMS would be expected to be
R = 4WaV[X] where I is the radiance of the surface (photons/(s sr cm2)), u is the photoionization cross section (cm2) of species X with concentration [XI (molecules/cm3),and V is the volume of
ANALYTICAL CHEMISTRY, VOL. 55, NO. 0, MAY 1983
872
0.5
i d
P 1
Ii
I
I
10.3m-
0.6..
i 1.0
j I
t.---7-----Drift Tlme (msec)
1.0-
I
4 Flgure 6. (a) Laser induced mobility spectrum of triethylamine at = 310 nm. The conditions under which this spectrum was recorded are as follows: T = 220 'C, input laser energy = 1.3 ml, laser beam size = 4 mm, electric field = 170.3 V/cm, triethylamine concentration = 250-300 ppb, drift gas flow = 600 cm3/min. K,(triethylamine) = 2.00. (b) Laser induced mobility spectrum of diisopropylamlne at X = 310 nm. The parameters are the Same as in (a): diisopropylamine concentration = 250-300 ppb, drift gas flow = 600 cm3/min. K,(diisopropyiamine) = 1.93. (c) Laser induced mobility spectrum of hexylamine at A = 310 nm. The parameters are the same as In (a): hexylamine concentration = 250-300 ppb, drift gas flow = 600 cm3/min. Note that no apparent signal is observed. (d) Laser induced mobility spectrum of hexylamine at X = 310 nm using a softly focused laser beam. The other parameters are the same as in (a): hexylamine concentration = 250-300 ppb, drift gas flow = 600 cm3/min. K,(hexylamine) = 1.95. (e) Laser induced mobility spectrum of hexylamine at 280 nm. The parameters are the same as in (a) except that the laser input energy = 0.4 mJ and hexylamine concentration = 250-300 ppb, drift gas flow = 600 cm3/min. K,(hexylamine) = 1.95.
the ionization region (cm3). The factor 4rI is for the case when the ionization region is surrounded by the radiating surface (27). In a typical system
